Steel stretches. Steel springs and steel rail cars stretch. The difference is a matter of degree. Pull on one end of a Slinky toy and the other end will move after a noticeable delay. The delay in the long train isn’t easily noticeable but it is there.

Sorry, George. Steel compresses too. Ski‘s idea of imagining the train made of rubber helps here as well. I’m sure you can visualize how rubber stretches and compresses - or does both at the same time when you bend it. (For a crude analogy, feel the skin gather inside and stretch outside of your elbow as you bend your arm.) The difference between the behaviors of steel and rubber in these examples is largely just a matter of degree. Look at that linked chart again. You’ll see measurements of both tension (stretching) and compression for various types of steel.

Makes sense. But why does SkiCarver think that this would happen at the speed of sound?

to be exact, its not the “speed of sound”, its “the speed of sound in x” where x is the material which the sound waves are travelling through. in terms of specifics of why it happens at “the speed of sound in x”, i know it does, however i am having difficulty in coming up with a cogent explaination. I will try to find some info.

It seems plausible to me but not a physicist so I’m probably getting to the edge of my understanding. Sound is waves of compression of a material. We typically think of a speaker in a radio and its quick movement bunching together some air molecules. That bunch, followed by others, propagates outward at a certain speed. I know that speed is different for different materials but I don’t know what the mechanism is that defines it. But it does seem reasonable that the same mechanism that limits the front of a series of sound waves might also limit the front of a single, continuously growing compression - such as a locomotive pushing on a line of rail cars.

It seems plausible to me but not a physicist so I’m probably getting to the edge of my understanding. Sound is waves of compression of a material. We typically think of a speaker in a radio and its quick movement bunching together some air molecules. That bunch, followed by others, propagates outward at a certain speed. I know that speed is different for different materials but I don’t know what the mechanism is that defines it. But it does seem reasonable that the same mechanism that limits the front of a series of sound waves might also limit the front of a single, continuously growing compression - such as a locomotive pushing on a line of rail cars.

Photons flout the light speed limit
17 August 2007
Mark Anderson
Magazine issue 2617
IT’S a speed record that is supposed to be impossible to break. Yet two physicists are now claiming they have propelled photons faster than the speed of light. This would be in direct violation of a key tenet of Einstein’s special theory of relativity that states that nothing, under any circumstance, can exceed the speed of light.

GÃ¼nter Nimtz and Alfons Stahlhofen of the University of Koblenz, Germany, have been exploring a phenomenon in quantum optics called photon tunnelling, which occurs when a particle slips across an apparently uncrossable barrier. The pair say they have now tunnelled photons “instantaneously” across a barrier of various sizes, from a few millimetres up to a metre. Their conclusion is that the photons traverse the barrier much faster than the speed of light.

Is this evidence that travel faster than the speed of light is possible?

I haven’t read about this experiment, but one of the conjectures I’ve seen within quantum mechanics is that such particles can go from one place to another without travelling the space between them. Something like a nano-wormhole idea.

Er, fair enough, but the wiki site claims, “discussion of the speed of gravity is normally in reference to general relativity, which predicts it to equal c.”

At the bottom of the wiki page, there is a link to a Usenet Physics FAQ website HERE (supported on the website of University of California, Riverside, Professor of Mathematical Physics John Baez, FWIW), that says the speed of gravity can potentially be calculated by measuring the gravitational damping of a pair of orbiting pulsars, “and the actual measurement confirms that the speed of gravity is equal to the speed of light to within 1%. (Measurements of at least one other binary pulsar system, PSR B1534+12, confirm this result, although so far with less precision.)”

Admittedly, that is from 1998, however the Wiki page cites other more recent experiments along the same lines.

The Metaresearch webpage you cite is at least somewhat questionable. THIS journalist piece writes about the webpage owner there, one Tom Van Flandern, and he does come across as somewhat crankish ... promoting ‘scientific ideas “outside of the mainstream of theories in Astronomy”’. Since he does have papers published in prestigious journals I am not going to dismiss him out of hand, however I think the Wiki page did the right thing in not giving his anti-Einsteinian approach to gravity much credence. At the very least much more experimental work would need to be done before jettisoning the central Einsteinian hypothesis that gravity propagates at c.

The word I used was ``may’‘. The word used before was ``is’‘. Do you reject data because it falsefies general relativity? There are lots of data that suggest the current LCDM model has problems.

It’s the way of science - new measurements suggest, even require, new models.

Well, yes, “may” ...

I rely on the general scientific consensus for questions like this. Every few weeks a new theory comes out, most of which do not stand the test of time. At this point, general relativity is still considered the best theory going, at least at the macro-level. We do also know that GR and quantum mechanics do not fit together well, in particular because there is as yet no adequate theory of quantum gravity. But until the scientific consensus fixes on a replacement for relativity, we are stuck with it. And its predictive capacity is amazing enough that whatever theory replaces Einstein will agree with it on most of the particulars.

I’m unsure what you are suggesting or what your interest is. My interest is to search for the Theory of Everything (TOE). Accordingly, the plethora of data that is accumulating that is inconsistent with the current ``standard’’ model is interesting as it points the way to the next model. Some published authors take issue with the idea GR is the ``general scientific consensus’’. J.V. Narlikar (the more outspoken on this) suggest (I think rightly) that GR is not a theory at all because it is only a calculating method and therefore, cannot be disproved. Mapping or transforming physically observable data into other, simpler mathematical coordinates is a common engineering and scientific practice. Therefore, ``predictive’’ capacity is really its capacity to be applied to many different physical models – not a model itself.

The question of whether ``curved space’’ or ``time dilation’’ is real is a philosophical issue. The more accepted model today is referred to as the Lambda Cold Dark Matter model (LCDM) is less than 10 years old, and perhaps younger depending on how you view the changes and challenges in the last 2 years. Before the LCDM, there was inflation and other major changes. It seems what is considered commonly accepted is undergoing major changes every few years. So far, even the published and opposing physical models use GR to calculate. So it seems the only model that has really stood the test of time is Newtonian or its experimental PPN.

Quantum and cosmological models do not fit together well on a great many more issues. One other is the quantum entanglement. Although proponents are careful to say the speed of light is not violated, in the end information about quantum states is transferred faster than light.